Dow Company presented [Me2Si(Me4C5)NtBu]TiCl2(Constrained-Geometry Catalyst, hereinafter abbreviated as CGC) in the early 1990's (U.S. Pat. No. 5,064,802), and the advantages of CGC in the copolymerization reaction of ethylene and alpha-olefin, compared to the previously known metallocene catalysts, can be summarized as follows: (1) it exhibits high activity even at high polymerization temperature, and yet, produces a high molecular weight polymer, and (2) it also has very excellent copolymerizability with alpha-olefins having large steric hindrance such as 1-hexene and 1-octene. In addition, as various properties of CGC in a polymerization reaction have been gradually known, there have been vigorous attempts to synthesize derivatives thereof and use as polymerization catalysts in academic and industrial fields.
A Group 4 metallocene compound having one or two cyclopentadienyl groups as ligand may be activated with methylaluminoxane or a boron compound to be used as a catalyst of olefin polymerization. Such catalyst exhibits unique properties that cannot be realized by a Ziegler-Natta catalyst of the prior art.
Specifically, a polymer obtained using the catalyst has narrow molecular weight distribution and better reactivity to second monomers such as alpha-olefin or cyclic olefin, and the distribution of the second monomers of the polymer is uniform. Also, by changing substituents of the cyclopentadienyl ligand in the metallocene catalyst, stereoselectivity of the polymer may be controlled when polymerizing alpha olefin, and a degree of copolymerization, a molecular weight, and the distribution of second monomers and the like may be easily controlled, when copolymerizing ethylene with other olefins.
Meanwhile, since a metallocene catalyst is expensive compared to the Ziegler-Natta catalyst of the prior art, it should have good activity so as to have economical value. If reactivity to second monomers is good, a polymer including many second monomers may be obtained even with a small amount of second monomers introduced.
Many researchers have studied on various catalysts, and as a result, it was proved that bridged catalysts generally have good reactivity to second monomers. Bridged catalysts studied till now can be classified into three kinds according to the shapes of bridges. The first is a catalyst wherein two cyclopentadienyl ligands are connected by an alkylene dibridge by the reaction of electrophile such as alkyl halide with indene or fluorene and the like, the second is a silicon-bridged catalyst connected by —SiR2-, and the third is a catalyst a methylene-bridged catalyst obtained from the reaction of fulvene with indene or fluorene and the like.
However, among these attempts, a few catalysts are practically applied in commercial plants, and there has been continued demand for the preparation of catalysts exhibiting more improved polymerization performance.